Optical devices such as photonic sensors, optically controlled switches, and wavelength converters often use waveguides for guiding and supporting the radiation signal. An advantageous technology that can be used for fabricating waveguide structures with different functionality is semiconductor-on-insulator (SOI) technology. A silicon optical waveguide consists of a silicon core (with high refractive index) and a cladding material with a lower refractive index. This cladding material is typically oxide, air, or polymer. These waveguides are fabricated on SOI (silicon-on-insulator) wafers using lithography, etching, and deposition process steps.
Optical waveguide complexes can be configured in such way that they allow filtering, splitting, modulating, or manipulating the incoming signal. One example of an integrated optical device is a directional coupler, which is used to transfer radiation from one waveguide to another waveguide. The directional coupler basically consists of two waveguides with the same width placed sufficiently close to each other so that optical power can be interchanged between the waveguides and incoming radiation, launched into a first waveguide, can couple into a second waveguide upon propagation through the directional coupler. A directional coupler can be designed to function as a simple splitter (e.g. 50/50 splitter or 10/90 splitter), but a directional coupler can also be used as a wavelength filter (e.g., splitting 1310 nm wavelength and 1490 nm wavelength) or as a polarization splitter (splitting Transverse Electric (TE) and Transverse Magnetic (TM) modes). Depending on specific design parameters such as the length over which the waveguides are in close proximity, the directional coupler can thus be used as a splitter, a wavelength filter or as a polarization filter.
The directional coupler thus can be a versatile building block for photonic integrated circuits, but its performance depends strongly on the width of the waveguides and the width of the gap in between both waveguides.
Efficient coupling between adjacent waveguide channels also plays an important role in optical ring resonators, which are optical devices comprising a waveguide in a closed loop (for instance a ring or a racetrack) coupled to one or more input/output waveguides. When radiation of an appropriate wavelength is coupled from the input waveguide into the loop, it builds up in intensity due to constructive interference over multiple circuits around the ring resonator. Ring resonators are wavelength selective devices that may be used for various filter and modulation applications.
Currently two different types of dielectric index-guided waveguides are used. Deep (etched) wave guides, also known as strip or wire waveguides, and shallow (etched) waveguides, also known as rib or ridge waveguides. The disadvantage of the shallow waveguide is that it has a high leakage loss for TM polarization, so it can only have low loss for TE polarization.
In most cases, wire waveguides are used to guide radiation from the input to the output of the coupled waveguide system. An example of such a deep etched waveguide or strip or wire waveguide is shown in FIG. 1a (cross section). The width w of the core 10 of the strip waveguide is typically in the order of 450 nm and the thickness t is typically in the order of about 220 nm. A disadvantage of the deep etched waveguide is that it is very sensitive to the dimensions, in particular to the linewidth of the waveguide. Furthermore, the performance of a coupling system based on deep waveguides also depends strongly on the widths of the waveguides and on the distance between the waveguides (gap width). Sidewall surface roughness of the wire waveguides can induce huge propagation losses and forms an obstruction to realize high-efficient optical coupling devices.
Rib waveguides, being less sensitive to surface roughness, can be used as well in radiation coupling devices, but have to deal with high propagation losses of guided TM modes due to coupling to leaky TE modes. An example of a shallow etched waveguide or rib or ridge waveguide is shown in FIG. 1b. In this example, the width W (unetched portion) is typically in the order of 650 nm, the thickness t is typically in the order of 220 nm and the height h (difference in thickness between the unetched portion and the shallow etched portion) is typically in the order of 50 nm. As indicated in I.E.E.E. Photonics Technology Letters, Vol. 19, March-April 2007, pages 429-431, “Width dependence of inherent TM-mode lateral leakage loss in silicon-on-insulator ridge waveguides”, by M. A. Webster, rib waveguides formed in silicon-on-insulator can be designed to have a reduced loss for TM-modes by carefully choosing the width of the waveguide.
Although tuning of the width W of the waveguide may solve the problem of making a shallow waveguide that guides a TM mode with low loss, this solution typically may not be suitable for all optical waveguide devices. For example in filters, the waveguide widths need to be adapted to the filter characteristic. The waveguide width that is needed for the particular filter property will in general not be the same as the waveguide width that is needed to have low TM losses.